Tech Tip: So High It Hertz!

The question: Why do preamps have frequency response levels way above the range of human hearing?

To answer this question, we have enlisted the expertise of audio design guru and founder of Millennia Music and Media, John La Grou. John’s intimate knowledge of analog circuit design makes him a regular guest lecturer and panel judge at AES (Audio Engineering Society) conventions, as well as one of the most respected designers in the industry. Millennia products are used by artists ranging from Barbara Streisand to the Rolling Stones; engineers ranging from Eddie Kramer (Jimi Hendrix) to Bruce Swedien (Michael Jackson); mastering studios including "the Bobs," Bob Ludwig of Gateway Mastering and Bob Katz of Digital Domain; symphony orchestras such as the New York Philharmonic and Los Angeles Philharmonic; music institutions including Juilliard and Eastman schools of music, and broadcast companies including NBC and CBS New York. There are over 30,000 Millennia channels in professional use. We’d like to thank Mr. La Grou for providing our readers with an exclusive inside look at the often-confusing realm of manufacturer’s specifications.

Us: It’s been pretty well established that an extended frequency range (well upwards of 20kHz) makes for a better-sounding preamp. The fact that there’s acoustic information above 20kHz that we can detect certainly justifies that thinking. But, when you factor in the 16-20kHz reach of most mics and monitors, as well as the stonewall limits of the Nyquist-Shannon theorem in digital recording, it appears that the wide bandwidth is doing something else for the audible range other than presenting "for-dogs-only" harmonic content. What’s really going on?

John La Grou: Excellent question. More times than not, it’s been our experience that analog audio circuits perform more transparently, with less coloration, more realistic space and depth, and better perceived dynamic realism, etc. when usable bandwidth is extended beyond 20kHz. How much extension is necessary? Based on extensive listening tests, we’ve developed a design rule that strives to maintain a 200-300kHz useable bandwidth on all circuits, which is at least an order of magnitude beyond normal human hearing limits. We’ve actually extended this bandwidth even further, but find diminishing benefit beyond 200-300kHz. Moreover, opening up usable bandwidth too far invites RF (Radio Frequency) and self-oscillation issues.

While it’s true that most microphones and speakers are design-limited to around 20kHz, we’ve found that wide-bandwidth electronics such as mic preamps, summing nodes, EQ, line amps, etc., maintain much better overall "in-band" program reality. A speaker may not reproduce beyond 20kHz, but we certainly hear the in-band improvements achieved using ultra-wide bandwidth electronics. Why? One reason may be phase performance. A signal that’s -3dB at 25kHz can exhibit a measured in-band phase shift of roughly -50 degrees (10kHz)! When the -3dB point is extended to 250kHz, in-band phase shift is reduced to less than 1 degree.

But there’s more. We can measure different circuits for static objective performance—THD (Total Harmonic Distortion) , FR (Frequency Range), etc. —and see no differences. Yet, when we compare these circuits with our ears, they can sound profoundly different. There are dynamic characteristics of audio circuits that we cannot yet measure objectively. While our test equipment shows no change in measured performance, our ears tell us otherwise. Dr. Earl Geddes and others have been working on characterizing these audible but heretofore immeasurable effects. We’re getting closer to objective test criteria, which fully characterize human perception, but we’re not there yet.

Us: Are their any other areas where you would design well beyond an accepted range for better sound reproduction?

JL: There’s a parallel design rule using large dynamic range power supply rails. Many analog electronics devices operate via relatively low-voltage rails, which means that big dynamic signals can often approach the limitation of the power supply, leaving little or no internal headroom. When an audio signal approaches the power supply limit, measurable and audible non-linearities can often result. To avoid this problem, we design with higher voltage rails; 50V-100V solid-state and 170V-330V vacuum tube. Internal audio signals can then operate well under any power supply limitation, thus avoiding supply-related non-linearities.